FORMAL definitions, folks.
Yes I know this but there has to be some logic that filters by those values. Where is it and how is it implemented. # I should just read the spec
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Each protocol has their own spec. Wanna send an email without an email client? Use telnet to connect to Port 25 on your email SMTP server. Use the smtp spec to find out what to type to send an email (usually a HELO command).
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You’re explaining me things I already know but you’re answering different questions. I know specs exist. How are they *implemented*
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Ports are essentially post office boxes. When the kernel receives a packet, it sorts the packet to the correct buffer by port number. If a program has informed the kernel it wants to listen on that port, then the kernel also wakes that program up to let it read the packet
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A metaphor is not a formal definition
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Fair. Would "it's a persistent identifier that enables routing within a machine" be what you're looking for? There's not a lot to formally define; they're routing numbers that, unlike IP addresses, don't require negotiation or cooperation between systems
End of conversation
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once a userland process asks to listen on a specific port, with the appropriate system calls to the kernel, the kernel looks out for any incoming TCP packet with only the SYN bit set and also having that numeric port number in the 16 bit “destination port” field in that packet
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RFC specifications. It's not a filter. RFC says that all ftp traffic should be on port 21, so all ftp servers by default listen on 21. You CAN tell your ftp server to use any other port you want though. You just have to tell the client side what the new port is.
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